Hypocaloric diets are the first, and many times the only, treatment for patients with obesity. The overall aim of this application is to define the optimal composition of a weight reduction diet for treatment of obesity. the specific aim is to investigate the effect of ketosis on protein and fat metabolism during weight reduction with very low calorie diets (VLCD's) (<600 cal/day). To achieve this end it is proposed: a) to test the hypothesis that protein sparing is enhanced if ketosis is prevented during weight reduction, b) to test the hypothesis that reduction in the rate of gluconeogenesis from amino acids is one of the mechanisms involved in protein sparing during weight reduction, c) to measure the relative contribution of glycerol and amino acids to gluconeogenesis as the duration of treatment is prolonged, and d) to test the hypothesis that the rate of lipolysis in vivo is a function of the degree of caloric restriction but not of the diet composition. Obese women (>30% overweight) will be randomly assigned to consume a 600 cal non-ketogenic diet (50 g protein, 75 g carbohydrate) or an isocaloric, isonitrogenous ketogenic diet for 28 days. The effect of ketosis on protein sparing will be assessed by measuring rates of leucine oxidation and gluconeogenesis, nitrogen balance and lean body mass (LBM). Rate of leucine oxidation and rate of gluconeogenesis from amino acid will be measured before and at day 14 and 28 by a primed continuous infusion of [1- 14C] leucine and a simultaneous infusion of [6-3H]-glucose and [U-14C]- threonine, respectively. Nitrogen balance and LBM determined by bioelectrical impedance will be measured daily. Indirect calorimetry and rate of lipolysis and the contribution of glycerol to gluconeogenesis will be measured before and at days 14 and 28 by a simultaneous infusion of [6- 3H]glucose and [U-14C]glycerol. Insulin, glucagon, and variable of safety (EKG, electrolytes, uric acid, liver and renal function tests) will be measured weekly. The results of this study will provide information of the metabolic perturbation provided by ketosis on protein and fat metabolism, and will form the basis for the design of rational dietary treatment of obesity.